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False Color Images and Multi-Wavelength Astronomy

'Why isn't the galaxy more colorful? The pictures of it that we've seen online are very very colorful' is one of the common complaints that I've gotten when we introduce students to the night skies. The concept of False color images, how they are recorded and how they are clubbed together to make the multi-colored pictures are what follow as my answer to that question.

Color, in the astronomical sense of the word, is defined as the brightness of a source in a particular filter. When astronomers talk about blue stars or red stars, they are speaking in qualitative terms that the star is stronger in the B filter than it is in the R filter, if one is using the UBVRI filters. Such filters encompass a broad range of wavelength, a few 100 nms usually. Quantitatively, emission can be characterized as continuum emission, such as blackbody radiation, and discrete emission i.e spectral lines. In the example mentioned above, if one mentions that a star is bluer, they are referring to the fact that the peak of the blackbody radiation, determined using the Wein's displacement law, is in the B filter. Synchrotron emission and Bremsstrahlung are two other major contributors to the continuum radiation from an astronomical object. In some cases, spectral lines are also important in determining the color of an astronomical object. Here are twoarticles that talk about false colors in detail.

So, having hopefully convinced you of false colors in astronomy, let's look at how our own milky way looks in different parts of the electromagnetic spectrum. These images are taken from a wide array of observatories and satellites that have instruments dedicated to observing a particular part of the EM spectrum.

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